The present invention relates to a stroboscopic scanning electron microscope.
In general, when electron beams are radiated onto an object to be examined, such as an LSI, secondary electrons are emitted. The number of secondary electrons varies with a surface potential of the LSI. This is called a voltage contrast phenomenon. When the number of secondary electrons generated at a surface portion of the LSI is measured based on the voltage contrast phenomenon, the potential of this surface portion can be measured. Thus, the stroboscopic scanning electron microscope utilizes a combination of the voltage contrast phenomenon and stroboscopic principles.
In general, the internal circuits of the LSI are synchronously operated in accordance with a synchronizing signal .phi.. As shown in FIG. 1, when the pulse electron beam radiates synchronously with the circuit operation at the phase .phi..sub.1, a secondary electron signal indicates the surface potential at the phase .phi..sub.1. When the surface of the LSI is two-dimensionally scanned to obtain the signal which indicates the number of secondary electrons, the potential distribution of the LSI at the phase .phi..sub.1 can be displayed as an image.
FIG. 2 shows a conventional stroboscopic scanning electron microscope. Referring to FIG. 2, reference numeral 11 denotes a stroboscopic scanning electron microscope which has a beam pulse generator 12; 13, an LSI as the object to be measured; 14, a tester for supplying a test signal to the LSI 13; and 15, a beam pulse generating circuit for controlling the beam pulse generator 12. An electron beam 10 is generated by the beam pulse generating circuit 15 in synchronism with a synchronizing signal SYNC from the tester 14. Reference numeral 16 denotes a secondary electron detector which comprises, for example, a scintillator and a photo-multiplier; 17, a CRT display unit for displaying with an image contrast the number of secondary electrons detected by the secondary electron detector 16; 18, a D/A converter for supplying a scanning control signal S.sub.X to an X-axis scanning coil 19 through a changeover switch S1 to deflect the electron beam 10, thereby scanning the object with the electron beam 10 along the X-axis; 20, a D/A converter for supplying a scanning control signal S.sub.Y to a Y-axis scanning coil 21 through the changeover switch S1 to deflect the electron beam 10, thereby determining the position of the beam spot of the electron beam 10 along the Y-axis; 22, a frequency divider for frequency-dividing the clock signal from the D/A converter 18; and 23 and 24, variable constant-battery circuits, DC voltage signals which are respectively supplied to the X- and Y-axis scanning coils 19 and 21 through the changeover switch S1.
The mode of operation of the conventional stroboscopic scanning electron microscope shown in FIG. 2 will be described with reference to the timing chart in FIG. 3. A test signal is supplied from the tester 14 to the LSI 13 which is then operated. In this condition, a synchronizing signal SYNC corresponding to the test signal is supplied from the tester 14 to the LSI 13. The synchronizing signal SYNC is also supplied to the beam pulse generating circuit 15. The beam pulse generator 12 is also controlled by an output from the beam pulse generating circuit 15. As a result, an electron beam 10 which has a period corresponding to the period of the synchronizing signal SYNC is radiated from the beam pulse generator 12 onto the LSI 13. The position of the LSI 13 which corresponds to the beam spot of the electron beam 10 is determined by the X- and Y-axis scanning coils 19 and 21. The X-axis scanning coil 19 is controlled by the scanning control signal S.sub.X from the D/A converter 18. The output from the D/A converter 18 has a stepwise waveform one step of which corresponds to one pixel, as shown in FIG. 3. The Y-axis scanning coil 21 is controlled by the scanning control signal S.sub.Y from the D/A converter 20.
However, in the conventional stroboscopic scanning electron microscope shown in FIG. 2, the scanning control signals S.sub.X and S.sub.Y are not synchronized with the electron beams 10. As a result, the number of pulses of the electron beams 10 varies at different positions of the LSI 13 radiated therewith. For example, three electron beams are radiated onto the LSI 13 during the period of a scanning control signal S.sub.Xl, whereas four electron beams are radiated thereonto during the period of a scanning control signal S.sub.X2. In this manner, when the number of beams radiated onto the LSI 13 differs, the corresponding numbers of secondary electrons for corresponding pixels differ. As a result, the measuring precision of the potential distribution on the LSI 13 is degraded.
Two observation modes are included in the observation of the LSI 13 to be described below. In the first observation mode, a pulsed electron beam is positioned on the desired node of the LSI 13, and a voltage waveform at the surface point of the LSI 13 is displayed by an oscilloscope. The first observation mode is called a waveform mode. In the second observation mode, an electron beam is scanned over the desired area of the LSI 13, and the number of secondary electrons from the LSI 13 is visually displayed on a CRT so as to show potential contrast images of the LSI obtained at a desired phase. The second observation mode is called an image mode using an SEM (scanning electron microscope). In the conventional stroboscopic scanning electron microscope shown in FIG. 2, the secondary electron signal from the LSI 13 is detected by the secondary electron detector 16. An output from the secondary electron detector 16 is then supplied as an analog signal to the CRT display unit 17. In the CRT display unit 17, the numbers of secondary electrons are displayed corresponding to the phases of voltage signal from the LSI, thereby obtaining potential contrast images.
On the other hand, when the electron beam 10 is radiated onto a given position of the surface of the LSI 13, the changeover switch S1 is operated to apply voltages preset by the variable regulated power supplies 23 and 24 to the X- and Y-axis scanning coils 19 and 21, respectively. The position onto which the electron beam 10 is radiated is determined by the voltages preset by the variable regulated power supplies 23 and 24. When the beam spot of the electron beam 10 is moved on the LSI 13 by operating the variable regulated power supplies 23 and 24, a spot 25 on the CRT display unit 17 is also moved. Therefore, in order to obtain a voltage waveform at the given point of the surface of the LSI 13 by the conventional electron microscope, the surface of the LSI 13 is scanned with the electron beam 10 to display the secondary electron image on the CRT display unit 17. Thereafter, the changeover switch S1 is switched to set the waveform mode. When the preset voltages of the variable regulated power supplies 23 and 24 are changed, the spot 25 on the CRT display unit 17 is moved to detect a desired observation point. It should be noted that an image 26 remains as an afterimage even if the image display state is changed to spot radiation since the CRT display unit 17 has an afterimage effect. For this reason, the spot 25 is moved while the observer is observing the image 26, so that the electron beam 10 can be radiated onto the desired position of the surface of the LSI 13.
However, in the conventional electron microscope, positioning must be performed for each of a plurality of observation points which are present on the LSI 13, resulting in cumbersome measurement. Furthermore, positioning for the observation points is performed using the afterimage obtained when the spot of the electron beam 10 is moved, thereby degrading the positioning precision. In addition to these drawbacks, the number of injected electrons per unit area on the LSI 13 increases when the CRT display unit 17 is set in a high magnification display mode. As a result, the operation of the LSI 13 is adversely affected. Furthermore, since the electron beam 10 is continuously radiated onto the LSI 13 while positioning for each observation point is being performed, the LSI 13 is greatly affected.